Aluminum base alloy



United States Patent M 1 2,915,390 ALUMINUM BASE ALLOY Charles B. Criner, Murrysville, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa, a corporation of Pennsylvania No Drawing. AppiicationJanuary 13, 1958 Serial No. 708,352

6 Claims. (Cl. 75-141) This invention relates to high strength aluminum base alloys and more particularly to certain compositions which retain a relatively high strength when heated and held at elevated temperatures.

The properties and performance of aluminum and aluminum base alloy products at room temperatu-reare well-known, and it is also known that castings and forgings of certain alloys have served Well in reciprocatingtype internal combustion engines for such parts as pistons, cylinder heads and cylinder barrels when the barrels are provided with a ferrous metal liner. With the development of high speed aircraft and other machines that are exposed to or develop high temperatures, there has been a demand for aluminum base alloys which will havea higher strength than the commercial compositions heretofore available. There has been a particular need for high strength alloys which can withstand aerodynamic heating for short periods of time. For such purposes, the temperature may not exceed 300 or 400 F. yet many commercial aluminum base alloys suffer a drop in tensile strength of 33% or more upon being heated to' 300" F. and a decreaseof more than 60% at. 400 F. as compared to the strength at room temperature. Furthermore, the same alloys possess a low resistance to creep at these elevated temperatures.

Itis an object of this invention to provide an improved aluminum base alloy which in the worked, solution heat treated and age hardened condition possesses an exceptionally high strength at 300 F. and. at higher temperatures up to 400 F.

Another object is. to provide an improved aluminum base alloy that has a high resistance to creep at temperatures up to 400 F.

Still another object is to provide an improved aluminum base alloy that has a higher modulus of elasticity than the present commercial aluminum alloys used for service at elevated temperatures.

These and other objects and advantages are achieved in a wrought alloy consisting essentially of aluminum, 3 to 9% copper, 0.15 to 1.0% manganese, 0.02 to 0.5 cadmium, 0.2 to 3% lithium, 0.05 to 2% magnesium and 0.1 to 2% zinc. To develop the desired strength at room temperature and at temperatures of 300 to 400 F. the alloy must be solution heat treated, quenched and age hardened at a temperature somewhat above room temperature. In this condition a wrought article of an alloy nominally composed of aluminum, 4.5% copper, 0.5%

manganese, 0.3% magnesium, 0.6% zinc, 1.2% lithium and 0.2% cadmium with small amounts of iron and silicon impurities may be expected to have a tensile strength of 77,000 p.s.i., a yield strength of 74,000 psi. and an elongation of about 12% at 300 F. after 100 hour exposure. In addition to possessing a high strength at temperatures up to about 400 F. the alloy has a high resistance to creep, on the order of 0.005 in./in. for a period of 100 hours at 300 F. under a stress of 60,000 p.s.i. The alloy is superior to the commercial aluminum base alloys that have been recommended for service at temperatures up to 400 F. and it also has a higher strength and resistance to creep at such temperatures than the same alloy without the magnesium and zinc additions.

At room temperature the alloy has a modulus of elas- 2,915,390 Patented Dec. 1, 1959 ticity on the order of 11.2)(10 which is higher than that of most aluminum base alloys.

To attain the desired properties, the above mentioned proportions for the several elements must be strictly observed. If less than 3% copper is present, the alloy will not havethe required strength while if more than 9% is used the alloy is not readily worked. The best results are obtained if the copper content is kept between 4.5 and 6%.

As to thev manganese component, at least 0.15% must be present to attain the desired strength at elevated temperatures but more than about 1.0% is objectionable as it interferes with working of the alloy.

Cadmium and lithium have a unique and unusual effect in combination with the other alloying elements in producingan alloy which has a high strength at temperatures-on the order of 300 to 400 F. The effect of cadmium and lithium is surprising in view of their relatively low melting points and their susceptibility to oxidation. Only a relatively small amount of cadmium is required, namely, 0.02 to 0.5%. Larger amounts are undesirable because of the possible presence of free or uncombined cadmium which is detrimental to the working characteristics of the alloy. The quantity of lithium used must also be restricted to a narrow range, no significant benefit being obtained if less than 0.2% is employed while there is no further improvement in properties at room and elevated temperatures if more than 3% is present.

Contrary to expectations, it has been found that the addition of both magnesium and Zinc to the aluminumcopper-cadmiumlithium-manganese base composition, set

forth above, is beneficial to the properties of the alloy at elevated temperatures. Heretofore, aluminum base alloys containing magnesium and zinc as primary alloy additions have been characterized by a rapid decrease in strength upon being heated to 300 F. or higher, but in combination with copper, manganese, lithium and cadmium the magnesium and zinc serve to increase the strength of the alloy at these elevated temperatures. T o attain this result, the magnesium should be employed in amounts of 0.05 to 2% and the zinc in the proportion of 0.1 to 2%. In preferred practice the magnesium content should fall within the range of 0.2 to 0.8% and the zinc content should lie between 0.2 and 1.5%. If more than 2% of magnesium or zinc is present, the strength at elevated temperatures decreases and the advantage gained from the presence of these elements is lost. It is significant that the addition of both zinc and magnesium is necessary and that the combination produces much higher strengths at room temperature as well as at temperatures up to 300 to 400 F. than is possible through separate additions of either element alone. In respect to the proportion of zinc to magnesium, when the amount of zinc used is less than 0.5%, the magnesium content may be less than, equal to, or exceed the quantity of zinc in the alloy. Where larger amounts of zinc are employed, the magnesium content should be less than that of zinc, preferably in the ratio of not more than one part of magnesium to two parts of zinc.

Although cadmium is preferred as the element to be used in the alloy, it may be replaced in whole or in part by one or more of the elements mercury, tin, indium, and thallium in amounts of 0.02 to 0.3% each, the total of these and cadmium not exceeding 0.75%. These elements and cadmium are therefore considered as constituting a group of selected metals, at least one of which must be employed in the alloy to achieve the desired properties referred to above. It should be understood, however, that in substituting one or more of the elements for cadmium, the strength and resistance to creep may not be as high as that obtained where cadmium is used,

but still these properties are superior to those found in the same alloy but devoid of magnesium and zinc.

Under some casting conditions it may be desirable to add certain well-known grain refining elements to the alloy to improve the cast structure. For this purpose it is advisable to employ at least one element selected from the group composed of 0.002 to 0.05% boron, 0.01 to 0.25% titanium, 0.02 to 0.3% zirconium and 0.01 to 0.1% vanadium; the total amount of such additives should not exceed 0.5%.

The silicon impurity content of the alloy may be as high as 0.6% without adverse effect upon the strength of the alloy at elevated temperatures. In preferred practice the silicon content is kept below 0.2%. The iron impurity should not exceed about 0.6%.

To attain the desired high strength the alloy should be in the wrought condition, i.e., it should have undergone such working operations as rolling, forging, extrusion or pressing in order to break up the cast structure of the ingot. The initial working is usually done at elevated temperatures and this may be followed by working at room temperature. In any case the amount of working will be determined by the size and shape of the final product.

As has been mentioned, the alloy must be solution heat treated and age hardened above room temperature. Generally, heating the wrought product for l to 12 hours at 920 to 980 F. is necesssary to cause substantially complete solution of the soluble constituents. Upon completion of the solution heat treatment, the article should be rapidly quenched from the elevated temperature to retain the benefits of that treatment. Following the quench, the product should be aged for to 200 hours at 275 to 350 F. The benefit of the thermal treatment is retained upon exposure of the alloy to elevated temperatures even though it might be expected that over-aging would occur with resultant loss in strength.

If desired, some articles made from the alloy which have received the foregoing treatment may be cold worked to gain a further improvement in strength.

It has been found that wrought products of the alloy which have received the above-mentioned thermal treatment can be heated repeatedly in service to temperatures as high as 300 to 400 F., without substantial change in strength of the alloy at elevated temperatures. This ability to withstand such heating and cooling cycles without substantial detriment is especially valuable where the alloy is employed as a skin covering for high speed aircraft which is subject to aerodynamic heating.

The high strength and resistance to creep which characterize the new alloy are illustrated in the following examples. For purposes of comparison an aluminumcopper-manganese-titanium alloy is included which has been proposed for elevated temperature service as well as one related to the present invention but containing no magnesium and zinc. The chemical composition of all the alloys is given in Table I below. Aluminum is considered as constituting the balance of the alloy in each case.

TABLE 1 Composition of alloys Per- Per- Per- Per- Per- Per- Per- Per- Per- Alloy cent cent cent cent cent cent cent cent cent Cu Mn Gd Li Mg Zn Fe Si Ti The alloys were melted, cast and forged to di ameter rods which were then cut to lengths suitable for later machining into tensile test and creep test specimens. Sections of forged bars of alloys B, C, D, E, F and I were solution heat treated at 960 F. while the bars of alloys G and H were treated at 940 F. In each instance they were soaked at temperature for 2 hours, quenched in cold water and finally aged by heating to 320 F. for 12 hours. The forged bars of alloy A were solution heat treated for 2 hours at 1000 F. quenched and aged 12 hours at 375 F. Tensile test bars and creep test specimens were machined from the sections of forged rod. The tensile test bars were divided into groups, one group being tested at room temperature and the others heated to 212 F. (except bars of alloy A), 300 and 400 F. for periods of /2 hour and hours and tested at temperature. The mechanical property values obtained in these tests are given in Table II below.

TABLE II Tensile properties at room and elevated temperatures AT ROOM TEMPERATURE Tensile Yield Elonga- Alloy Strength, Strength, tion, Perp.s.i. p.s.i. cent 100 Hours Alloy Tensile Yield Elong., Tensile Yield Elong., Strength, Strength, Percent Strength, Strength, Percent p.s.l. p.s.i. p s 1 p.s.i.

AT 300 F.

M Hour 100 Hours Alloy Tensile Yield Elong., Tensile Yield Elongn, Strength, Strength, percent Strength, Strength, percent p.s.i. p.s.i. p.s.i. p.s.l.

AT 400 F.

l Hour 100 Hours Alloy Tensile Yield Elong., Tensile Yield Elong., Strength, Strength, percent Strength, Strength, percent p.s.l. p.s.i. p.s.l. p.s.l.

It is apparent that the alloys containing magnesium and zinc possess a much higher strength than alloy A at room temperature, 212 F. and at 300 F. Also, the now composition has higher tensile and yield strength values at these temperatures than the alloy B containing no magnesuim or zinc. This superiority, although less marked, is also evident at 400 F. The decrease in strength and increase in elongation produced by the 100 hour exposure at 400 F. indicates that it is not advisable to continuously expose the alloys at such a temperature for such a long period of time if one wishes to take advantageof the high strength characteristic of these compositions. It is to be noted that the presence of 0.53% silicon in alloy I did not impair the strength of the alloy.

Solution heat treated and aged test bars of alloys B, C and G were heated to 300 and 400 F. and stressed sufliciently to cause a creep of 0.001, 0.002, 0.005 and 0.01 in./in. and eventual rupture. The periods of time and the stress values for each increasement of creep are given in Table III. The values for alloy B are the average for that composition and others almost identical with it.

TABLE III Creep and stress rupture properties at 300 and 400 F.

AT 300 F.

STRESS REQUIRED FOR TOTAL CREE]? AFTER INDICATED TIME Stress for Alloy Time 0.001 0.002 0.005 0.01 Rupture at (Hrs.) 1n./in. in./in. in./in in./in. Indicated Time 0.1 65,000 67,000 69,000 69,000 70,000 1 55,000 62, 000 65,000 65, 000 65, 000 B 45, 000 54, 000 58,000 58, 000 58, 000 100 36,000 46, 000 50, 000 50, 000 50, 000 1, 000 29. 000 38, 000 44. 000 44, 000 000 0.1 68,000 69,900 1 65,000 67,000 69, 000 72, 000 C 10 59, 000 63,000 66, 000 67, 000 67, 000 100 54, 000 57, 000 60, 000 61, 000 61, 000 1,000 51,000 54,000 54,000 54,000 0.1 1 62, 000 72, 000 G 10 49, 000 61, 000 66,000 66, 000 66, 000 100 52,000 59,000 60,000 60,000 1,000 ,000 53,000 ,000

AT 400F.

STRESS (P.S.I.) REQUIRED FOR TOTAL CREE]? AFTER INDICATED TIME Stress Time 0.001 0.002 0.005 0.01 (p.s.i.) for Alloy (Hrs) tn./in. in./tn. inJin. in./in. Ruptureat Indicated Time 0.1 50,000 53,000 55,000 56,000 56,000 1 36, 000 46, 000 49, 000 50, 000 50, 000 B 10 19, 000 32. 000 38,000 39,000 39, 000 100 9, 500 16, 000 26, 000 28, 000 29, 000 l, 000 5, 500 10, 000 18, 000 ,000 21, 000 0.1 50,000 53,000 56, 000 57,000 58,000 1 36, 000 46, 000 51,000 52,000 58, 000 0 10 19, 000 32,000 42,000 43, 000 45, 000 100 9,500 ,000 26, 000 30.000 33,000 1, 000 5, 500 10,000 18, 000 20, 000 23, 000 0.1 50,000 53,000 56,000 57,000 58,000 1 36, 000 46, 000 51, 000 52. 000 53, 000 G..... 10 19, 000 32, 000 42, 000 43, 000 45, 000 100 9, 500 16,000 26, 000 30, 000 33, 000 l, 000 5, 500 10, 000 18,000 20, 000 23, 000

It is apparent from the above test data that the alloys containing magnesium and zinc have a greater resistance to creep and a higher rupture stress at 300 F. than the alloy Without magnesium and zinc. At 400 F. the advantage over alloy B is not as marked, and under some conditions there appears to be no improvement. This suggests that in service there may be no advantage in using the alloys at 400 F. or higher temperatures.

Having thus described my invention and certain embodiments thereof, I claim:

1. A wrought aluminum base alloy consisting essentially of aluminum, 3 to 9% copper, 0.15 to 1.0% manganese, 0.02 to 0.5% cadmium, 0.20 to 3% lithium, 0.05 to 2% magnesium and 0.1 to 2% zinc, said alloy when solution heat treated at 920 to 980 F. and aged at 275 to 350 F. being characterized by a higher tensile and yield strength at 300 F. than the same alloy devoid of magnesium and Zinc.

2. An alloy according to claim 1, having a copper content of 4.5 to 6%.

3. An alloy according to claim 1 having a magnesium content of 0.2 to 0.8% and a zinc content of 0.2 to 1.5%.

4. An alloy according to claim 1 wherein the ratio of magnesium to zinc does not exceed 1 part of magnesium to 2 parts of zinc when the zinc content exceeds 0.5

5. A wrought aluminum base alloy consisting of alumi num, 3 to 9% copper, 0.15 to 1.0% manganese, 0.02 to 0.5% cadmium, 0.2 to 3.0% lithium, 0.2 to 2% magnesium, 0.1 to 2% zinc and at least one element selected from the group composed of 0.002 to 0.05% boron, 0.01 to 0.25% titanium, 0.02 to 0.3% zirconium and 0.01 to 0.1% vanadium the total amount of said group of elements not exceeding 0.5%, said alloy when solution heat treated at 920 to 980 F. and aged at 275 to 350 F. being characterized by a higher tensile and yield strength at 300 F. than the same alloy devoid of magnesium and zinc.

6. A wrought aluminum base alloy consisting essentially of aluminum, 3 to 9% copper, 0.15 to 1.0% manganese, 0.20 to 3% lithium, 0.05 to 2% magnesium, 0.1 to 2.0% zinc and at least one element of the group consisting of cadmium, mercury, tin, indium and thallium in amounts of 0.02 to 0.5% cadmium and 0.02 to 0.3% of the other individual elements of the group, the total amount of said elements not exceeding 0.75%, said alloy when solution heat treated at 920 to 980 F. and aged at 275 to 350 F. being characterized by a higher tensile and yield strength at 300 F. than the same alloy devoid of magnesium and zinc.

References Cited in the file of this patent UNITED STATES PATENTS 2,026,551 Fink Jan. 7, 1936 2,381,219 Le Baron Aug. 7, 1945 2,579,369 Dawe Dec. 18, 1951 2,784,126 Criner Mar. 5, 1957 UNlTED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,. 2,915,390 December 1, 1959 Charles B. Criner ears in the -printed specification It is hereby certified that error app rection and that the said Letters of the above numbered patent requiring cor Patent should read as corrected below.

Column 1, line 50, for "0.5" read 005% column 5, line A, for 00.0w" read new same column 5, Table III, in group C, fourth column thereof, under the heading "0.002 in /in sixth line, for "69,900" read Signed and sealed this 17th day of May 1960 (SEAL) Attest:

KARL H. AXLINE Attesting Ofiicer ROBERT C. WATSON Commissioner of Patents 

1. A WROUGHT ALUMINUM BASE CONSISTING ESSENTIALLY OF ALUMINUM, 3 TO 9% COPPER, 0.15 TO 1.0% MANGANESE, 0.02 TO 0.5% CADMIUM, 0.20 TO 3% LITHIUM, 0.05 TO 2% MAGNESIUM AND 0.1 TO 2% ZINC, SAID ALLOY WHEN SOULATION HEAT TREATED AT 920 TO 980*F. AND AGED AT 275 TO 350*F. BEING CHARECTERIZED BY A HIGHER TENSIL AND YIELD STRENGTH AT 300*F. THAN THE SAME ALLOY DEVOID OF MAGNESIUM AND ZINC. 